The present invention relates to a novel formulation for a fracturing fluid for use in fracturing subterranean formations such as oil and gas wells.
It is known to utilize an emulsion consisting of water or water with a polymer therein as the continuous phase, and from about 50% to about 80% oil (crude or refined) as the discontinuous phase, with a surfactant such as sodium tallate or a quaternary amine in the aqueous phase. This is shown in Polymer Emulsion Fracturing (Sinclair et al), a 1973 publication of the Society of Petroleum Engineers. It is also known, as shown in U.S. Pat. No. 4,233,165, to utilize a water-in-oil emulsion as a fracturing fluid. In that case, a surfactant soluble in oil is utilized to maintain an aqueous internal phase in a continuous diesel phase.
Moreover, stable foams have been developed where the external phase is water and the internal phase is CO2 and the foam is stabilized by selective chemical foamers. It has been found that the internal phase (CO2) in such foam has to be a minimum of 53%. The Western Company of North America has developed two foams, the first one where a combination of CO2 and Nitrogen is used to create a stable foam with the addition of chemical foamers. The combined internal phase of gas is still over 52% by volume. The second foam involves stabilizing the CO2 foam with a crosslinker without the use of foamer. By using less CO2 the produced gas from a well can be put on a pipe-line faster without flaring for a long time to meet pipeline CO2 content regulations. Moreover, all the above systems use gelling agents from guar to CMHPG to stabilize the foams. There have also been attempts made to foam CO2 in diesel or crudes with limited success because of the natural antifoam ability of these oils.
Conventionally, instead oil gels have been used. However, with the potential for downstream problems of the phosphate esters used in gelling oils, there is a need for alternatives. Attempts have been made to pump CO2 with gelled oils, mainly to use the energy of the phase change from liquid to CO2 gas to clean up wells efficiently. However, most phosphate ester based oil gels are not compatible with CO2; when these systems are made to survive under CO2 conditions, breaking with conventional breakers for these systems, which tend to be high pH buffers tend to be neutralized by the CO2 or in the presence of any water, will chelate the aluminum or iron crosslinkers (complexers) and cause precipitation resulting in formation change.
The approach of the present invention is to provide an emulsion of oil in liquid CO2. This is possible with the selection of an appropriate surfactant, in particular methoxy-or ethoxy nonafluorobutane. This approach provides an emulsion of high sustained permeability, without the need or gelling and breaking, and the costs associated therewith. The amount of CO2 required is less than 48%, which results in cost effectiveness, and the inherent ability of CO2 simply to gasify and escape to the atmosphere indicates simplified clean-up after fracturing.
The object of the present invention, is to provide a fracturing fluid consisting of an emulsion of hydrocarbon in a liquified gas such as liquid CO2.
A further object of the present invention is to provide an emulsion of liquid CO2 in which is dissolved a surfactant, and in which are suspended droplets of a hydrocarbon fluid, for use in fracturing oil and/or gas well formations.
In a broad aspect, then, the present invention relates to a fracturing fluid consisting of an emulsion having a continuous phase of a liquified gas, a discontinuous phase of a hydrocarbon, and a surfactant soluble in the two phases.